The present invention generally relates to a method and an apparatus for measuring the thickness of a thin film and more particularly, relates to a method and an apparatus for measuring the thickness of a thin gate oxide layer accurately before moisture and organic residue are deposited on the film to cause erroneous readings.
In the fabrication process for semiconductor devices, the process of growing a gate oxide layer for insulating a gate is an important step. In ULSI fabrication, the thickness of the gate oxide layer grown has been reduced to less than 100 xc3x85 by using the 0.35 xcexcm technology. The process control for growing such thin gate oxide layers is therefore more critical than those used in the 0.7 xcexcm technology for growing gate oxide thicker than 200 xc3x85. To grow the ultra-thin gate oxide layers, particle, organic and metal contaminations must be reduced in a super-clean room technology and with improved cleaning processes. It has been found that in order to improve the integrity of ultra-thin gate oxide layers, the surface of a silicon wafer must be free of native oxide or other contaminants.
When the fabrication technology progresses into 0.1 xcexcm, the thickness of the gate oxide layer may well be under 50 xc3x85. At such small thickness, the surface micro roughness at the SiO2/Si interface also becomes an important factor on channel electron mobility as well as other gate oxide qualities, for instance, the breakdown voltage of the gate oxide layer. Different contaminants may cause different detrimental effects on the device reliability when the gate oxide integrity is in question. The problems are important since gate oxide quality is one of the critical steps that determine the yield, reliability and performance of a ULSI circuit. Problems that may occur due to roughness, impurity and contamination can be the result of insufficient cleaning technology, poorly controlled oxidation technology and how the silicon wafer was prepared. Although various cleaning processes have been developed to remove contaminants, an important consideration is to avoid contamination rather than to clean it up during processing.
The ultra-thin gate oxide layers used in ULSI devices can be formed by many different techniques. One of such techniques is a rapid thermal oxidation process for forming an ultra-thin gate oxide layer of 60xcx9c70 xc3x85 for a 0.25 xcexcm technology, or a thickness of 40 xc3x85 or less for a 0.18 xcexcm technology. In a rapid thermal oxidation process, the equipment for performing the oxidation is similar to that used in a rapid thermal processing technique so that a process chamber can rapidly increase the temperature of a wafer, rapidly changing various gas requirements in the chamber and achieving a high vacuum without causing contamination to the wafer surface. In a rapid thermal oxidation process, the gate oxide formation can be carried out at a temperature between 950xc2x0 C. and 1200xc2x0 C. with reasonable growth rates.
In order to accurately control the quality of the ultra-thin gate oxide layer formed, the thickness of the layer grown must be accurately monitored. The monitoring or measuring of the ultra-thin gate oxide thickness becomes more important as the film thickness becomes smaller with the 0.25 xcexcm or the 0.18 xcexcm technology. Since the gate oxide layer is transparent at such small thickness and is formed over a highly absorbing substrate of silicon, a technique of ellipsometry is frequently used to determine the film thickness. While ellipsometry is used to determine the thickness of thin transparent dielectric layers by utilizing a visible light source, semiconductor layers that are transparent only to infrared light source can also be measured by using infrared. It has been found that for very thin semiconductor layers, i.e., such as in the ultra-thin gate oxide layers, even visible light penetrates deep enough for useful ellipsometric measurements to be made.
The ellipsometry operates by the principle that when an incident beam is plane polarized, the two perpendicular components will have different amounts of phase shift during reflection and therefore different reflection coefficients. The ellipsometry is usually used for the measurement of films of a thickness that is less than one wavelength of the viewing light. When ellipsometry is used on greater thicknesses as an interferometry, multiple number of thickness may have the same ellipsometric data.
The basic arrangement of an ellipsometer optics 10 is shown in FIG. 1. The optics 10 includes a monochromatic light source 12, a filter 14, a polarizer 16, a quarter wave plate 18, a specimen holder 20, an analyzer 22 and a detector 24. The polarizer 16, the analyzer 22 and the quarter wave plate 18 can all be rotated independently and their angular position with respect to the instrument frame closely monitored. By using appropriate initial settings of the three optical elements, namely the analyzer, the polarizer and the quarter wave plate, and then rotating the quarter wave plate and the analyzer until a light transmission minimum is observed, the various parameters required for calculating the film thickness can be determined. When different light wavelengths is used for the measurement, different quarter wave plates 18 must be used since the thickness of the plate must be tailored to the specific wavelength.
While the ellipsometer shown in FIG. 1 generally provides a reliable technique for measuring the thickness of an ultra-thin gate oxide layer, problems in obtaining accurate measurements are frequently encountered which are not related to the ellipsometric technique. For instance, it has been observed that after an ultra-thin gate oxide layer, i.e., about 20 xc3x85, is formed on a silicon wafer, the thickness measurement continuously increases with time. It has also been found that when ultra-thin gate oxide film of 20 xc3x85 is formed on a silicon wafer, the maximum deviation measured from its supposed thickness is about 0.4 xc3x85, or about a 2% deviation. It is therefore impossible to measure the real thickness of the ultra-thin gate oxide layer by the traditional ellipsometric technique when the thickness measured is time dependent.
In modern IC devices where the thickness of a gate oxide layer is extremely small, i.e., between about 20 xc3x85 and about 50 xc3x85, the measurement problem presents a serious drawback in the quality control of the devices. The cause for the continuing thickness increase on the ultra-thin gate oxide layer has been attributed to moisture and organic residue absorption on the gate oxide film surface, instead of any further oxide growth. The traditional ellipsometer therefore cannot be reliably used to monitor the thickness of an ultra-thin gate oxide layer.
A typical time-dependent measurement curve obtained on an ultra-thin gate oxide layer by a conventional ellipsometer is shown in FIG. 2. Data plotted in FIG. 2 are obtained in three separate tests on similar samples. It is seen that within the first 6 hours of deposition, thicknesses measured by the ellipsometer increase continuously from about 19.8 xc3x85 to about 20.2 xc3x85, resulting in a 0.4 xc3x85 increase, or approximately a 5% deviation from the original thickness measurement of 19.8. The film thickness further increases after 6 hours to approximately 20 hours, even though at a slower rate, to a final thickness of about 20.5 xc3x85. Such variations in the thickness measurements cannot be tolerated for reliability reasons.
It is therefore an object of the present invention to provide a method for measuring a thickness of a thin film that does not have the drawbacks and shortcomings of a conventional measurement technique.
It is another object of the present invention to provide a method for measuring a thickness of a thin film that has a surface sensitive to moisture or organic residue.
It is a further object of the present invention to provide a method for measuring a thickness of a thin film by a modified ellipsometric technique.
It is another further object of the present invention to provide a method for measuring a thickness of a thin film that has a surface sensitive to moisture and organic residue by first heating the thin film and evacuating an enclosure wherein the thin film is situated to a temperature between about 400xc2x0 C. and about 800xc2x0 C., and to a sub-atmospheric pressure.
It is still another object of the present invention to provide a method for measuring a thickness of a thin film of less than 50 xc3x85 wherein the film has a surface sensitive to moisture and organic residue.
It is yet another object of the present invention to provide an apparatus for measuring a thickness of a thin film that has a surface sensitive to moisture and organic residue by providing a heating chamber connected to a thickness measuring device wherein the heating chamber can be evacuated to form a vacuum therein.
It is still another further object of the present invention to provide an apparatus for measuring thickness of a thin film that has a surface sensitive to moisture and organic residue by providing a heating chamber capable of heating a film to a temperature between about 400xc2x0 C. and about 800xc2x0 C. at a sub-atmospheric pressure.
It is yet another further object of the present invention to provide a method for measuring a thickness of a gate oxide layer not thicker than 100 xc3x85 by an ellipsometer which includes the step of first heat treating the gate oxide layer to a temperature not higher than 800xc2x0 C. in a chamber at a pressure of not higher than 760 Torr for at least 10 seconds.
In accordance with the present invention, a method and an apparatus for measuring a thickness of a thin oxide layer that are capable of producing accurate results are provided.
In a preferred embodiment, a method for measuring thickness of a thin film layer that has a surface sensitive to moisture and organic residue can be carried out by the operating steps of positioning a substrate that has a film layer thereon in a vacuum heating chamber, heating the substrate and the film layer to a temperature between about 400xc2x0 C. and about 800xc2x0 C. under a pressure of less than 760 Torr for at least 10 seconds in the vacuum heating chamber, positioning the substrate in a thickness measuring device within 10 minutes after removal from the vacuum heating chamber, and measuring a thickness of the film layer.
In the method for measuring a thickness of a thin film layer that has a surface sensitive to moisture and organic residue, the film layer may have a thickness less than 50 xc3x85, and preferably a thickness less than 25 xc3x85. The film layer may be a gate oxide layer that has a thickness of less than 50 xc3x85. The heating step may be carried out at a temperature of at least 500xc2x0 C. for a time period of at least 20 seconds. The method may further include the step of measuring the thickness of the film layer by a ellipsometer. The method may further include the step of transporting a substrate from a vacuum heating chamber to a thickness measuring device through an interior passageway of a conduit isolated from the atmosphere. The method may be a real time thickness measuring technique.
The present invention is further directed to an apparatus for measuring thickness of a thin film that has a surface sensitive to moisture and organic residue which includes a heating chamber capable of providing a temperature between about 400xc2x0 C. and about 800xc2x0 C. at a sub-atmospheric pressure, and a thickness measuring device positioned juxtaposed to the heating chamber such that a substrate heat treated in the heating chamber may be transported into the thickness measuring device within 10 minutes after completion of a heat treatment step in the heating chamber.
The apparatus for measuring a thickness of a thin film that has a surface sensitive to moisture and organic residue may further include a conduit connecting and providing fluid communication between the heating chamber and the thickness measuring device such that a substrate may be transported from the heating chamber to the thickness measuring device isolated from the atmosphere. The apparatus may further include an air evacuation means in fluid communication with a cavity in the heating chamber for reducing a pressure in the cavity to less than 760 Torr. The substrate may be a silicon wafer that has a less than 50 xc3x85 thick gate oxide layer formed on top. The thickness measuring device may be an ellipsometer. The apparatus is effective in transporting the film and the substrate into a thickness measuring device substantially without moisture and organic residue formed on top.
In an alternate embodiment, the present invention is directed to a method for measuring a thickness of a gate oxide layer not thicker than 100 xc3x85 by an ellipsometer which can be carried out by the operating steps of positioning a silicon wafer that has a gate oxide layer formed on top in a chamber, the gate oxide layer may have a thickness less than 100 xc3x85, heat treating the silicon wafer to a temperature not higher than 800xc2x0 C. in the chamber at a pressure less than 760 Torr for at least 10 seconds, transporting the silicon wafer to an ellipsometer within 10 minutes after removal from the chamber, and measuring a thickness of the gate oxide layer prior to deposition of moisture and organic residue on the gate oxide layer.
The method for measuring a thickness of a gate oxide layer not thicker than 100 xc3x85 by an ellipsometer may further include the step of transporting the silicon wafer through a passageway in a conduit that is substantially isolated from the atmosphere. The gate oxide layer may have a thickness less than 50 xc3x85. The method may further include the step of heat treating the silicon wafer to a temperature of between about 500xc2x0 C. and about 700xc2x0 C. for at least 20 seconds. The method may further include the step of measuring a thickness of the gate oxide layer in real time. The method may further include the step of heat treating the silicon wafer to a temperature not higher than 650xc2x0 C. and at a pressure not higher than 1 Torr.